An auxiliary power unit, (APU), is a type of gas turbine engine which is commonly mounted in aircraft and performs a number of different functions. These functions include providing secondary power to the aircraft as well as providing pressurized bleed air for main engine starting and the aircraft's environmental control system. Typically, APUs are started by a DC starter motor which is mounted on the APUs gearbox.
Referring to FIG. 1 which shows a prior art start circuit 10, electric starting of APUs in airplanes is often accomplished by providing DC power from two distinct sources in parallel. One source is an onboard APU battery 12 and the other source is DC power derived from AC generators through a transformer-rectifier unit 14, (TRU). When an APU start is initiated, commands from the aircraft control system close contactor 16 and starting current from the APU battery 12 and TRU 14 combine at junction 20 and then flow to the APU starter motor 26 provided contactors 2 and 24 are closed.
The operation of the circuit 10 is controlled by the APUs electronic control box, (ECB) 28. When the ECB 28 receives a command 50 from the aircraft to start the APU, it sends a close signal to contactors 22 and 24. Voltage sensors 32, 34, and 36 provide voltage signals to the ECB 28.
A disadvantage to this prior art circuit 10, is that when both the battery 12 and TRU 14 are operating, the combined inrush current is more than required to accelerate the APU. This occurs because the system is typically sized for battery only starting to assure that the APU can be started when the TRU is not operating. That is, for example, when the aircraft is on the ground and the main engines are shut down. Because the starter motor 26 is typically a series wound device, the combined power produces high inrush current at the inception of a start. This high inrush current can cause excessive heating of the starter motor, reduced life of the contactors, and reduced life of the APU gearbox due to the initial high impact torque generated by the starter motor.
Accordingly, there exists a need for a start circuit used in the electric starting of an APU aboard an aircraft that can mitigate high inrush current when starting power is being provided from a battery and TRU in parallel.